1. Field of the Invention
The present invention relates to an image processing method, a computer-readable program, an image processing device, an image forming device, and an image forming system.
2. Description of the Related Art
For example, an ink-jet printing device in which a droplet ejection head is used for the printing head is known as one of image forming devices including a printer, a facsimile, a copier and a multi-function peripheral. This ink-jet printing device is provided to eject the ink from the printing head to a sheet as the printing liquid, so that image formation is carried out on the sheet. The sheet in this case is not limited to paper, but it is meant to include an OHP (overhead projector) sheet, a printing medium, a recording medium, a recording sheet, a printing sheet, etc.
In order to allow the ink-jet printing device to spread from the personal use to the office use, it is necessary to resolve the two problems which follow.
One problem is the printing speed. In a commonly used ink-jet printing device (except for the special industrial type), the printing head which is smaller in size than the recording sheet is used to perform printing by scanning the sheet surface repeatedly and ejecting ink drops thereto. This is the line printing method. When compared with an image forming device of electrophotographic printing type which performs printing on a sheet (page) basis, the line printing method is disadvantageous in respect of the printing speed.
In order to obviate the problem of the printing speed, some improved methods are adopted which include the increase of the scanning speed by raising the cycle of ejection of ink drops, the reduction of the number of times of the scanning by enlargement of the printing head size or by the bidirectional printing, and the shortest scan control which performs scanning only to the areas in which the image data are printed. With such improved methods, the printing speed that is higher than that of the electrophotographic printing method may be realized in the case of printing of a small or middle number of sheets.
The other problem is related to the cost, and the formation of an image on plain paper by the ink-jet printing device with lost cost is usually difficult. If a special sheet for ink-jet printing is used, high quality image reproduction may be attained. The image quality of the ink jet printed image created by the ink-jet printing device of the personal use in recent years has come to be increasingly close to that of a photograph.
However, such special sheets are generally expensive, and the introduction of the special sheets into companies or offices in which severe cost control is required is difficult. In addition, high-level image quality is not required for the images outputted for the office use, and if the high-level image quality can be attained only with the special sheets, the ink jet printing is disadvantageous in respect of the cost.
In order to allow the image formation on plain paper, it is necessary to improve the composition of the ink. For example, development of dye ink with low permeation, use of auxiliary fixing agent, development of pigment ink, etc. have been tried. The newest model of the ink-jet printing device has come to perform the image formation on the plain paper commonly used in offices or the sheets commonly used as copy papers with good image quality which is equivalent to that of the image forming device of electrophotographic printing type.
The ink-jet printing device is becoming a very attractive product also in the office use by the improvement of the scanning speed and image quality. In particular, it is advantageous in respect of the cost when compared with a laser printer. And the miniaturization of the ink-jet printing device is easy and the desktop use is also progressing.
However, unlike image forming devices, such as laser printers and offset printing devices, which include the mechanism for fixing the coloring material to the sheet surface, the ink-jet printing device makes the coloring material permeate into the sheet. The ink-jet printing device has the problems or restrictions accompanied with the permeation process.
One of the problems is that the ink of one color previously adhered to the sheet colors more conspicuously than the ink of another color subsequently adhered to the same part of the paper. The ink-jet printing device for the office use makes use of the bidirectional printing in which printing is performed in each of the forward and backward motions of the printing head, in order to raise the printing speed. However, when the bidirectional printing is performed, the order of impact of ink will be reversed for the forward printing and the backward printing, and a different coloring for every scanning line of the forward printing and the backward printing may take place. As the phenomenon, the thin horizontal-stripe patterns appear on the image being originally formed. Thus, the image quality will deteriorate.
Some countermeasures against the color difference (bidirectional color difference) accompanied with such bidirectional printing have been proposed. One of them is to use a head unit having a symmetrical arrangement in which the nozzles of the respective colors are arranged to be symmetrical to the main scanning direction. For example, see Japanese Laid-Open Patent Application No. 11-320926 and Japanese Laid-Open Patent Application No. 07-242036.
With the head unit of the symmetrical arrangement, the two nozzles in the head unit are shifted along the scanning line by one pitch of the resolution and the images formed sequentially in order of the forward printing and the backward printing are arranged per every 1-dot line. Theoretically, no color difference is produced by the coloring itself.
However, some problems remain unresolved in realizing the head unit of the symmetrical arrangement, and it is very difficult to realize the arrangement in which the nozzles of the respective colors are arranged to be completely symmetrical.
For example, if the head composition is provided to use separate units for the respective colors, the four color heads of cyan (C), magenta (M), yellow (Y), and black (K) will require seven or eight units. The cost in such a case will be increased more than that in the case of the head unit of the asymmetrical arrangement.
Especially, in the case of the printing head of the type in which the diaphragm which constitutes a part of pressurizing liquid chamber deforms according to displacement of a piezoelectric element to eject ink by the pressure by the volume change, the miniaturization of the head unit itself is difficult. With the increase in the number of head units, this will result in the enlargement of the head module and the enlargement of the carriage, i.e., the enlargement of the ink-jet printing device.
Moreover, when assembling the respective units, care must be taken to the assembling accuracy of not only the units of different colors but also the units of the same color. This causes the difficulty of manufacture to become high.
In the case of the printing head of the type in which sudden heating of the ink contained in the pressurizing liquid chamber is performed, so that the ink is ejected by using the volume change by the film boiling, it is easy to manufacture the printing head of symmetrical composition as an integral unit by applying the photolithographic printing method. And it is possible to reduce the difficulty of assembling. However, the recovery mechanism for keeping the state of the nozzle surface constant must be provided commonly to the respective colors, and a color mixture on the nozzle surface is likely to take place.
In recent years, in order to raise the image quality of black characters, the use of inks of different characteristics, like dye inks used as color inks and a pigment ink used as a black ink, is increasing. And not only the color mixture due to the recovery mechanism deteriorates the image quality, but also nozzle clogging due to the solidified sub-products etc. leads to a possibility of lowering the ink ejection stability.
To overcome the problem, the head unit of the asymmetrical arrangement in which the nozzle for ejecting the black ink is separated from the other nozzles of the symmetrical arrangement has been proposed as disclosed in Japanese Laid-Open Patent Applications No. 2005-001336 and No. 2005-001337.
However, the separation of the black nozzle from the other color ink nozzles, as disclosed in Japanese Laid-Open Patent Applications No. 2005-001336 and No. 2005-001337 causes the problem of the bidirectional color difference between the black ink and the other color inks to occur again, which must have been eliminated by the symmetrical arrangement.